This invention relates to NZP type structures of high permeability suitable for use in filtration applications such as in diesel particulate filtration.
Refractory NZP bodies are suitable for use in high temperature applications due to their high melting point and low coefficient of thermal expansion.
Present methods of making such bodies are disclosed in U.S. Pat. Nos. 4,675,302, 4,751,206, 4,801,566, 4,925,816, 5,102,836, 5,128,288, 5,254,510, and 5,488,018. Such bodies are expected to have low permeability due to low porosity or fine pore size; or in other cases, to have porosities and pore sizes so large that the bodies would be expected to exhibit poor filtration efficiency in removal of particles from a gas stream.
A need therefore exists to have low thermal expansion refractory NZP-type bodies with higher permeability than has been achieved up to now, while at the same time with useful filtration efficiency.
The present invention provides such a body and a method of making and using it.
In accordance with one aspect of the invention, there is provided a structure made predominately of an NZP-type phase having the general formula RxZ4P6xe2x88x92ySiyO24, where 0xe2x89xa6xxe2x89xa68, 0xe2x89xa6yxe2x89xa66, R is Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Y, and/or lanthanides, and Z is Zr, Ti, Nb, Ta, Y, and/or lanthanides, and optionally a sintering additive. The structure has an open porosity of at least 20% by volume, a median pore diameter in micrometers equal to or greater than a value defined by the quantity [10xe2x88x920.10(% open porosity)], both as measured by mercury porosimetry, and four-point modulus of rupture as measured on a solid rod of circular cross section, of at least 300 psi.
In accordance with another aspect of the invention, there is provided a method of making the structure that involves forming a mixture of NZP-forming raw material powders that are metal oxide sources capable of reacting to form an NZP-type reaction product, and/or pre-reacted powder having the above general formula. The volumetric average of the median particle diameters of the raw material powders in the raw materials combination is at least 15 micrometers, and the particle diameters of at least 65% of the totality of the raw material particles are greater than 11 micrometers. The mixture is shaped into a green structure, and fired to produce the finished structure.
The structure is preferably multicellular, e.g. a honeycomb, and most preferably an alternately plugged honeycomb suitable for use as a diesel particulate filter.
In accordance with another aspect of the invention, there is provided a method of removing particulates from a fluid stream that involves passing the stream through an alternately plugged honeycomb to retain the particulates in the pores of the honeycomb.